Has Dune been discovered?

Titan, where Kurt Vonnegut’s Sirens sang, is, as we all know, a foggy world shrouded in hydrocarbons. The Huygens probe that sank to its surface revealed a tantalising glimpse of its strangeness, with possible erosion by liquid methane rivers and sediments of icy substances. But Huygens didn’t really tell us much, like the probe that lasted a few minutes on equally obscure Venus. To map a foggy world you need orbital radar. The Cassini mission, the mother ship for Huygens, carried a high-resolution radar imaging system, and the results are astonishing; Titan has monster sand dunes (Lorenz, R.D. and 39 others 2006. The sand seas of Titan: Cassini RADAR observations of longitudinal dunes. Science, v. 312, p. 724-727). They dwarf all but the largest terrestrial dunes in Namibia, rising to 200 m. They are linear dunes, spaced at around 4 km, and trend parallel to Titan’s Equator, where there must be a wind belt. So far only a few images have been returned, so the extent of the dune systems is unknown.  However, they correlate with optically dark material that is extensive in the equatorial region, so Titan may be dominated by dunes. For dunes to form presupposes an abundant supply of particles small enough to be picked up and transported by winds. The images from different latitudes suggest that transport is equatorwards. What those particles are made of is impossible to tell from radar returns, but most likely they are either organic solids or ice. Notions of Titan being bathed in hydrocarbon oceans now fall flat, as the areas that are not dunes seem to be topographic highs.


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